The present invention relates generally to carbon brushes for use as current collectors in dynamoelectric machines and, more particularly, to a method of treating blanks for such brushes and the brushes resulting therefrom, such brushes exhibiting enhanced wear characteristics particularly at high temperature and low humidity.
Carbon current collection brushes are employed in rotating brush-type machines in which the brush blanks are generally fabricated of carbon, a relatively poor electrical conductor, reinforced by other materials and graphitized. For example, carbon is mixed with a pitch binder and the mixture is graphitized. The blanks are then assembled into brushes. These brushes are referred to as carbon current collection brushes or merely carbon brushes.
The above types of carbon brushes are retained in position by brush holders which generally are in the form of square, rectangular or cylindrical sleeves serving as a guide for any radial motion of brush resulting from vibration or eccentricity of the armature (commutator or slip ring) and brush wear. The brush holder may be mounted on a bracket to maintain a rigid position spaced from the commutator surface. An adjustable spring, connected to the bracket, bears on the top surface of the brush to maintain the desired contact pressure of the bottom surface of the brush on the commutator segments or slip ring.
The wear rate of a carbon brush in a dynamoelectric machine increases very rapidly with increasing temperature. For example, the life of a typical carbon brush under a standard load of 100 amperes per square inch (APSI) for a brush wear of 250 mils, decreases from about 8,500 hours at 100.degree. C. to about 700 hours and 200.degree. C. An important contributing factor to increased brush wear at elevated temperatures is direct oxidation of the carbon brush surface at the sliding contact interface leading to loss of carbon as gaseous oxides. This oxidation is catalyzed by the presence of copper in the commutator or slip ring. Minute particles of copper oxide migrate along the basal planes of the graphite crystallites into the interior of the brush producing a catalytic effect that leads to enhanced gasification rates and to increased brush porosity. Various approaches have been employed in attempts to overcome rapid brush wear, including the use of lubricants.
One method of increasing brush life by decreasing wear is found in U.S. Pat. No. 3,841,906 "Method of Treating a Carbon Current Collection Brush Blank and Brush Resulting Therefrom" by Albert L. Grunewald et al., issued Oct. 15, 1974, which patent is assigned to assignee of the present invention and which patent is hereby specifically incorporated by reference. The above patent describes a treatment method and resultant brush with enhanced wear characteristics, particularly at high temperature and low humidity. The method defined by the Grunewald et al. patent calls for impregnating a brush blank by contacting that blank with a solution of zinc naphthenate and then curing the blank by air drying and heating to remove solvents and retain zinc naphthenate in the blank in the amount of approximately four to six percent by weight of the total blank.
The brush described and made in accordance with the method of U.S. Pat. No. 3,841,906 is superior in many respects to an untreated blank. It has been found, however, to exhibit a relatively high coefficient of friction between the brush and motor commutator (or slip ring) particularly under low load, low current operation of the dynamoelectric machine. This high friction condition can result in "chattering" of the brush in its holder with resultant poor commutation (including sparking) which could ultimately result in damage to the machine part, particularly the commutator or slip ring.